Completed Precourse-1

Exercise_1.py

@@ -2,19 +2,30 @@ class myStack:
   #Please read sample.java file before starting.
   #Kindly include Time and Space complexity at top of each file
      def __init__(self):
+        self.stack = []
 
      def isEmpty(self):
+        return len(self.stack) == 0
 
      def push(self, item):
+        self.stack.append(item)
 
      def pop(self):
-
+        if self.isEmpty():
+            return None
+
+        return self.stack.pop()
 
      def peek(self):
+        if self.isEmpty():
+          return None
+        return self.stack[-1]
 
      def size(self):
+        return len(self.stack)
 
      def show(self):
+        return self.stack
 
 
 s = myStack()

Exercise_2.py

@@ -6,10 +6,21 @@ def __init__(self, data):
 
 class Stack:
     def __init__(self):
+        self.top = None
 
     def push(self, data):
-
+        new_node = Node(data)
+        new_node.next = self.top
+        self.top = new_node
+
     def pop(self):
+        if self.top is None:
+            return None
+
+        popped = self.top.data
+        self.top = self.top.next
+
+        return popped
 
 a_stack = Stack()
 while True:

Exercise_3.py

@@ -3,6 +3,8 @@ class ListNode:
     A node in a singly-linked list.
     """
     def __init__(self, data=None, next=None):
+        self.data = data
+        self.next = next
 
 class SinglyLinkedList:
     def __init__(self):
@@ -17,16 +19,78 @@ def append(self, data):
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
+        new_node = ListNode(data)
+        if self.head is None:
+            self.head = new_node
+            return
+        current = self.head
+        while current.next:
+            current = current.next
+
+        current.next = new_node
 
     def find(self, key):
         """
         Search for the first element with `data` matching
         `key`. Return the element or `None` if not found.
         Takes O(n) time.
         """
-
+        current = self.head
+        while current:
+            if current.data == key:
+                return current
+            current = current.next
+        return None
+
+
     def remove(self, key):
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+        if self.head is None:
+            return
+
+        # If the head is the node we want to remove
+        if self.head.data == key:
+            self.head = self.head.next
+            return
+        current = self.head
+        while current:
+            if current.next.data == key:
+                current.next = current.next.next
+                return
+            current = current.next
+
+    def print_list(self):
+        current = self.head
+
+        while current:
+            print(current.data, end=" -> ")
+            current = current.next
+
+        print("None")
+
+
+# Test
+linked_list = SinglyLinkedList()
+
+linked_list.append(10)
+linked_list.append(20)
+linked_list.append(30)
+linked_list.append(40)
+
+print("Original list:")
+linked_list.print_list()
+
+print("\nFinding 20:")
+node = linked_list.find(20)
+print(node.data if node else None)
+
+print("\nRemoving 20:")
+linked_list.remove(20)
+linked_list.print_list()
+
+print("\nRemoving head (10):")
+linked_list.remove(10)
+linked_list.print_list()